Ambient occlusion from high to low poly
One cause of these artifacts may be hidden, but "set as to render", objects. If some of their faces might overlap to your active object during the baking.

Even your high poly object can cause this if you don't correctly enable the option "bake selected to active".



Example:



Low and highpoly



Object are in the same location, so faces overlap themeselves or are occluded



Image above is the baked with with the option disabled



This is if it is enable. You must select the highpoly, and THEN the lowpoly in order to get this result.

Someone asked for a moon? :-)